The present invention relates, in general, to a liquid crystal display, and more particularly to a method for fabricating a liquid crystal display using a top gate polysilicon thin film transistor (hereinafter "TFT") as a switching element and wherein source and drain electrodes are formed of an n.sup.+ polysilicon and a metal.
In general, a liquid crystal display is structured to have a liquid crystal material injected between two glass substrates with electrodes. As the optical properties of liquid crystal are changed when applying a voltage to the electrode, an image is shown in the liquid crystal display.
Such liquid crystal display may be broadly divided into simple matrix operating types and active matrix operating types. The simple matrix operating type employs the method a scanning voltage utilizing time divisional operation and a data signal voltage. On the other hand, in the active matrix type, each picture cell which has an independent active element (switching element) is independently operated, so that the influence caused by neighboring picture cells can be minimized, thereby enhancing the contrast and increasing the number of scanning lines.
In such an active matrix operating type, a thin film transistor is employed as an active element. The thin film transistor may be categorized as amorphous silicon TFT and polysilicon TFT, in accordance with the substance employed. In addition, thin film transistor are divided into stagger type, co-planar type, self-aligned type, top gate type and the like, according to shape.
Hereinafter, description for the picture cell structure of liquid crystal display employing a conventional top gate type polysilicon TFT as a switching element is to be given with reference to the annexed figures.
Referring to FIGS. 1A through 1G, there are shown conventional fabricate processes for a gate top polysilicon TFT liquid crystal display.
Firstly, a transparent glass substrate 1 is entirely covered with a buffer layer 2 which is an insulating film superior in adhesiveness and on which a semiconductor layer 3 is subsequently formed, as shown in FIG. 1A.
Subsequently, using a photo etch process, the semiconductor layer 3 is removed selectively so as to form an active layer 3a which is left on only the TFT-forming region, as shown in FIG. 1B.
FIG. 1C illustrates that a gate insulating film 4 and a metal layer 5 are deposited over the entirety, in due order.
Next, an etch process is applied to the metal layer 5 and the gate insulating film 4 to selectively remove portions of them, forming a gate electrode 5a on the active layer 3a, and forming a gate line 5b as shown in FIG. 1D. The gate line 5b is to operate the TFT of another picture cell.
As shown in FIG. 1E, using the gate electrode 5a as a mask, n type ions are, as indicated by arrows, implanted at a high density dose in the active layer 3a. An annealing treatment results in the formation of source/drain regions 6a and 6b.
FIG. 1F shows the formation of an interlayer insulating film 7 having contact holes 8. For this, the interlayer insulating film 7 is deposited over the entire resulting structure of FIG. 1E and then, subjected to a selective etch process to form the contact holes 8 on the source region 6a and the drain region 6b.
Finally, as shown in FIG. 1G, a transparent electrode 9 is formed on a picture cell region of the TFT, followed by the formation of source/drain electrodes 10 on the source/drain regions. The source/drain electrodes are formed by depositing a metal such as aluminum and by applying a photo etch process to the metal so selectively as to leave it in only the contact hole regions. While the drain electrode interconnects the drain region 6b with the transparent electrode 9, the source electrode interconnects the source region 6a with a data line (not shown).
Such conventional liquid crystal display is operated as follows: application of a voltage greater than a threshold voltage to the gate electrode 5a lets a channel form at the interface between the gate insulating film 4 and the active layer 3a; during the formation of the channel, if a signal voltage is applied to the data line, the source region 6a is electrically conducted into the drain region 6b to transmit the signal voltage to the transparent electrode 9.
The liquid crystal display fabricated by the above conventional method shows such several problems as follow: first, since the source/drain electrodes 10 are connected with the source/drain regions 6a and 6b through the contact holes 8, respectively, it is difficult to control the contact resistance between the source/drain regions and the source/drain electrode; second, while the steps of depositing aluminum, coating a photosensitive film, exposing and developing are undertaken, the electrolyte of a developing solution may flow along the hillock of the aluminum deposited to come to contact with the transparent electrode 9 so that when the etching process is applied to the aluminum in order to form the source/drain electrodes 10, the transparent electrode 9 may be etched simultaneously with galvanic effect, resulting in breakage and an incomplete pattern of the transparent electrode 9; third, since the source/drain electrodes 10 are formed along with the data line which is made of single aluminum layer, the data line may be broken, so that the device reliability may be lowered.